This invention relates to a packet data communication system suitable for transferring data between a large number of mobile terminals and a central control station. The system is particularly suited for a satellite based system, where the central control communicates with the terminals via an orbiting satellite, but it could be applied to ground based systems.
There are many situations where it is desirable to transfer data between a control station and a plurality of distributed mobile terminals. For example, in the trucking industry, the truck""s on-board monitoring and logging devices may automatically report usage and location information back to a central station. Also, information relating to the environmental conditions, condition of the load, for example, or the temperature of refrigerated containers, can be reported. The operator may need to send schedule or routing changes to the drivers. Paging systems require text messages to be sent to individual customers. Traditionally these systems have carried relatively small amounts of data and do not require wideband channels. The bandwidth requirements, however, can change from time to time depending on the type and amount of data that needs to be transferred. Clearly, for example, voice channels will require a wider bandwidth channel than one that merely communicates position or environmental information.
Typically, SCPC (a Single Carrier per Channel) techniques are employed for such systems. In SCPC, as the name implies, each active channel is assigned a single narrow band carrier, typically not more than 20 KHz wide. The assigned channel depends on availability and desired bit rate. Bit rates are typically 0.6, 4.8, 19.2, kilobits per second. When the control station wishes to send a message to the terminal, the targeted terminal is notified over a very low bit rate control channel that it should expect to receive a message on a particular frequency. The targeted terminal then tunes to that frequency in order to receive and decode the incoming message.
In order to provide a sufficient number of channels, the maximum channel bit rate is typically limited to 19.2 kilobits per second. This means that such systems are not capable of carrying high quality voice signals, which typically require at least 32 kilobits per second. Such systems are generally not suitable for handling graphic information such as web pages, which is becoming an increasingly important application.
The need to assign a message channel over a control channel before passing the message also introduces a delay, known as latency, which makes it difficult to conduct interactive communications, for example, with the keyboard in real time.
An object of the invention is to provide a system that alleviates these disadvantages.
According to the present invention there is provided a packet data communication system having a control station and a plurality of remote terminals that communicate on demand with said control station over a wireless link, said control station comprising a data port for receiving data packets destined for said terminals; means for generating a plurality of data channels for carrying said data packets; means for assigning said data packets destined for a particular terminal to one or more of said data channels; means for generating a control channel carrying control information pertaining to said data channels; and means for transmitting said carriers to said mobile terminals as an r.f. signal; and each of said terminals comprising a receiver for receiving said r.f. signal; an analog-to-digital converter for digitizing said received signal; a buffer for storing said digitized received signal; means for monitoring said control channel to extract control information therefrom; and means for processing said stored signal to extract said packet data destined for said terminal from one or more of said data channels in response to control information received on said control channel.
Normally the received signal will be downconverted and demodulated to baseband prior to analog-to-digital conversion, although if desired with high speed processors it is contemplated that the entire processing could take place in the digital domain.
The remote terminals can either be fixed or mobile.
This system has the advantage of flexibility. Data packets can be sent on one channel or distributed simultaneously over several channels depending on the bandwidth requirements. For low bit rates, a single channel can be shared among several terminals. Unlike the prior art, it is not necessary for the channel assignment information to be sent in advance of the message. The assignment information on the control channel and the message can be sent simultaneously since the raw incoming data is stored. Only minimal processing is required to extract assignment information unless the control channel indicates that a message is addressed to the terminal in question, in which case the message can be extracted from the assigned carrier(s).
The carriers are preferably generated in a digital signal processor (DSP), which carries out the channel assignments. The channels are then transmitted by means of the r.f. carrier, normally via satellite, to the destination terminals.
The aggregate channels on the r.f. carrier are transmitted as frames bounded by predetermined time instants. Each buffer typically stores one frame of information. Each frame can contain multiple packets distributed across multiple channels.
The control information informs a particular terminal that the current frame contains a message for that terminal, as well as the channel assignments, and time and frequency reference information. Only minimal processing is required at the terminals to monitor the control channel since this has a very narrow bandwidth, typically 600 b.p.s. No attempt is made to decode the data unless a message is received from the control channel that data is present for the terminal in question.
Each terminal also contains a DSP, which on receipt of a control message decodes the currently stored frame to extract data packets destined for that terminal. If the data contains wideband information, such as graphics, the packets are likely to be distributed sequentially over several channels. The DSP will extract the packets from the various channels in accordance with the information received on the control channel and arrange them in the appropriate order before outputting them to the data processing circuitry.
The signal processing is preferably carried out with an advanced DSP, such as the TMS320C60.
The system is thus capable of generating and demodulating simultaneous multiple carriers within the sampling bandwidth. Per frame adaptive processing of multiple carriers is achieved through buffered data and sequential processing.
The system can support multiple bit rates and power levels. It can also support multiple return access methods, such as unslotted ALOHA, slotted ALOHA and assigned channel. It can be used for mobile dispatch services employed text and canned messages, medium length messages with low latency, as well as for TCP/IP connections which provide LAN extension, FT protocol, email and database transactions.
The invention also provides a method of establishing communication between a control station and one or more of a plurality of mobile terminals over a wireless link, comprising the steps of generating a plurality of carriers; dynamically assigning one or more data carriers to a destination terminal; modulating said one or more carriers with packet data for said destination terminal; generating a control carrier containing information pertaining to said modulated carriers; transmitting said data carriers and said control carrier as an aggregate signal to said destination terminal; buffering said aggregate signal at said destination terminal; extracting said control information from said aggregate signal; and extracting data from said buffered signal in response to a received assignment in said control information.
The invention also provides a system for improving data throughput by utilizing a bandwidth manager. The bandwidth manager monitors network parameters and compares current parameters with statistical data stored in a database. Heuristic rules are used to decide what changes need to be made to the parameters to optimize throughput.